Hardness of water (Classifications and removal)
Water which cannot form lather easily with soap is known as hard water and this characteristic is called hardness.
On the other hand, water which forms lather with soap is called soft water.
The hardness which can be removed easily just by boiling is temporary hardness, otherwise, it is called permanent hardness. Permanent hardness can be removed through some appropriate physical or chemical methods.
The principle ions responsible for hardness are calcium (Ca2+) and magnesium (Mg2+). Smaller quantities of other ions which cause hardness of water are iron (Fe2+), manganese (Mn2+), strontium (Sr2) and aluminum (Al3+).
Hardness of water causes many problems. The reaction between hard water with soap produces sticky, gummy deposit known as ‘soap curd’, a dirty ring around the bathtub. Hard water, thus consumes lot of soap unnecessarily.
When hard water is heated, calcium carbonate (CaC03) and magnesium hydroxide [Mg (OH) 2] precipitate out of the solution, forming rock like scale.
The scale, in turn clogs hot water pipes and the efficiency of water-heaters, boilers is reduced day by day, with increased amount of deposit. The cleaning of such equipments is also costly.
Due to reduced efficiency, the heat transmission goes down increasing energy consumption and also the equipment cannot last long. The unnecessary consumption of such soap, along with formation of scaling, makes hard water unsuitable for washing of clothes at home and in laundry.
Groundwater generally is harder (~ 300 mg/1) compared to surface water (~ 200 mg/1) and is termed as ‘soft’ water. Hardness is measured in mg/1 as CaC03. The total hardness is taken as the sum of Ca2* and Mg2+ as they are the main components causing hardness.
Hardness can be classified into two groups:
(i) Carbonate hardness:
In this case, the captions Ca2+ and Mg2 are associated with two anions C03 2” and HC03~
(ii) Non carbonate hardness:
The captions Ca2+ and Mg2+ are associated with CP and S04 2“, P04 3“, etc.
The carbonate hardness is temporary hardness and it can be removed easily by just boiling the water and is very important in the sense that the boiling leads to formation of carbonate causing scaling.
For the removal of hardness two processes can be adopted (1) Lime soda process (2) Ion exchange process.
Lime Soda Process:
In this process, quick lime (CaO) or hydrated lime (Ca [OH]2) is added to hard water. The soluble bicarbonate of (Ca2+) is precipitated out as insoluble carbonate and that of Mg2* as hydroxide.